compiler: integrate ir package into compiler package

The ir package has long lost its original purpose (which was doing some
analysis and optimization on the Go SSA directly). There is very little
of it left, which is best integrated directly in the compiler package to
avoid unnecessary abstraction.

compiler/errors.go

@@ -14,7 +14,7 @@ import (
 // makeError makes it easy to create an error from a token.Pos with a message.
 func (c *compilerContext) makeError(pos token.Pos, msg string) types.Error {
 	return types.Error{
-		Fset: c.ir.Program.Fset,
+		Fset: c.program.Fset,
 		Pos:  pos,
 		Msg:  msg,
 	}